The present invention relates to a superconducting tape and in particular to a composite superconducting tape.
The invention has been developed primarily for use with high load current carrying cables and will be described hereinafter with reference to that application. It will be appreciated, however, that the invention is not limited to this particular field of use.
It is known to use superconducting tapes to make coils, magnets, transformers, motors and generators as well as current carrying cables. Superconductors are materials which exhibit, at sufficiently low temperatures, substantially zero electrical resistance for direct current. The temperature at which a substance becomes superconducting is called the transition or critical temperature. Early superconductors had critical temperatures that necessitated refrigeration with liquid helium. However, due to low thermodynamic efficiency, this is an expensive an generally uneconomic arrangement. More recently it has been known to utilise xe2x80x9cceramicxe2x80x9d superconductors with critical temperatures up to around 135K.
Superconductors with critical temperatures over 77K are of particular interest as they can be operated with liquid nitrogen refrigeration at relatively high thermodynamic efficiency and relatively low cost.
Superconductors can only carry so much current before the voltage drop per unit of length begins to increase in an exponential form. The so called xe2x80x9ccriticalxe2x80x9d current, Ic, is defined for precision as the maximum current that can be carried before the voltage drop increases above 1 microvolt per centimeter.
Tapes comprising superconducting material, and referred to as superconducting tapes, are already known, and comprise one or many superconducting filaments in a medium of silver or silver alloy. The main class of superconducting tape are known as powder-in-tube or PiT tape. These are made by drawing or otherwise reducing a tube of silver or silver alloy filled with a powder form of the superconducting oxide or its precursor. The tubes are also rolled or otherwise formed into a thin tape.
Multifilamentary tapes are mostly made by grouping filled tubes in a common silver or silver alloy sheath at art intermediate stage of reduction.
One important superconducting oxide is known as Bi-2223, and is a compound oxide of bismuth, strontium, calcium and copper. Certain limited substitutions can be made, as required. Bi-2223 can also be considered a cuprate salt.
Known tapes usually have a thickness of between around 0.2 mm and 0.3 mm, and a width of between 2 mm and 5 mm. The superconducting filaments must be thin to obtain an adequate critical current and are typically around 10 to 40 microns in thickness. Moreover, the filaments typically have an aspect ratio of at least 1:10.
The filaments comprise many plate-like grains and, for good performance, the grains should be aligned as much as possible in the same crystallographic orientation. The relative orientation is often referred to as the grain alignment or xe2x80x9ctexturexe2x80x9d. Well textured filaments allow a high critical current, and give overall flexibility to the whole tape. If the filaments are too thick, then they will degrade at relatively large bending radii of the tape.
A superconducting tape""s flexibility can be measured in terms of the reversible bending radius achievable when such bending is carried out at room temperature. The reversible bending radius may be defined as the largest bending radius which causes more than 5% degradation in the critical current, Ic, as measured on the unstrained tape. Other criteria which are used to define the quality of the tape and the amount of current that a superconducting tape can carry is the critical current density, Jc, and the engineering current density, Jc. The critical current density, Jc, is defined by Jc=Ic/Ascxe2x88x92 where Asc is the total superconductor area in the cross-section of the tape, and the engineering current density Jc is defined by Jc=Ic/Atape, where Atape is the total cross-sectional area of the tape. The higher Jc and Jc the better. The ratio of the cross-sectional area of the superconductor to the cross-sectional area of the whole tape is called the Fill Factor, FF, and, therefore, we can deduce that FF=Jc/Jc.
When tapes are used to carry alternating current, rather than direct current, the superconducting tapes do not exhibit zero power loss, though losses are small compared to those exhibited by normal metallic conductors. This power loss resulting from the carrying alternating current rather than direct current is called xe2x80x9cAC lossxe2x80x9d.
For an individual tape, the AC loss can be of the order of 100 microwatts per meter per Amp of critical current squared. To achieve the maximum performance from these tapes the AC losses should be minimised. This is particularly true when they are used in superconducting cables where the current loads are high.
Composite tapes are sometimes made by forming a stack of individual tapes and wrapping the stack with one or more flexible wrapping tapes to keep it together. These wrapping tapes are generally a metal and more often silver.
Inevitably there are some gaps and/or overlapping between the turns of the wrapping tape. If such a wrapped stack were to be rolled then the gaps or overlapping would create kinks in the filaments, which destroys local grain alignment leading to degradation of the overall critical current density Jc.
It is an object of the present invention, at least in the preferred embodiment, to overcome or substantially ameliorate one or more of the disadvantages of the prior art, or at least to provide a useful alternative.
According to a first aspect of the invention there is provided a composite superconducting tape comprising a multiplicity of stacked and diffusion bonded superconducting tapes in which all elongate components extend longitudinally and a compatible metal tape is bonded to at least one exposed major surface.
The preferred embodiments of the invention making use of the combination described above to achieve a tape of high superconductor cross-sectional area and A thin and wide filament structure. This is highly desirable for a high critical current density.
Preferably, the filament thickness is as thin as is required to provide a desired critical current density, wherein the fill factor of the tape is about 40%.
Preferably also, the composite tape consists solely of the stacked and bonded constituent tapes. The composite tape has exposed major surfaces which are defined by those surfaces of the constituent tapes which are not abutted against an adjacent surface. More preferably, the exposed major surfaces of the composite tape covered with a metal. More preferably, the tape is of silver or silver alloy. In other embodiments, metal tapes other than those of silver are used.
In a preferred form, the superconductor filaments are at least 10 xcexcm away from the exposed major surfaces. More preferably, use is made of the thin metal tape to allow this to be achieved without incorporating unnecessary amounts of silver elsewhere in the composite tape.
Preferably also, the metal tape is used to control mechanical stresses. More particularly, some composite tapes are intended, in use, to be curved always in one direction such that one of the major exposed surfaces will always be convex and the opposed major exposed surface will always be concave. In such circumstances a metal tape is applied to the major surface that will be convex. More preferably, the major side that will be concave either does not include a metal tape or includes a second metal tape that is weaker than the first metal tape. In some embodiments the stronger tape is a silver alloy tape and the weaker tape a pure silver tape. In other embodiments the tapes differ in thickness.
Preferably, the metal tape is flat has a width not substantially greater than that of the superconducting tapes. In some embodiments the metal tape has a width of less than the superconducting tapes. In stilt further embodiments a wider metal tape is used which is, or subsequently becomes, bent to a channel section. The latter arrangement has structural advantages but adversely affects fill factor. Other embodiments utilise a silver foil or other compatible material which is wrapped around the stack but extending longitudinally.
Preferably, the constituent tapes are stacked in a single stack or in two or more parallel adjacent stacks. The latter normally requires one or two full width metal tapes to bridge from stack to stack. However, in some embodiments that bridging is achieved by one or two full width superconducting tapes. Structures with exactly two parallel stacks appear from preliminary experiments to have superior AC loss characteristics.
Preferably also, diffusion bonding between the superconducting tapes and metal tapes, if present, is obtained by assembling the tapes face to face and then heat treating them at a temperature at which diffusion between the metals occurs. However, it has been found that the temperature should be well controlled to minimise any deleterious effect on the superconducting material.
Good diffusion bonding between silver occurs at temperatures above 600xc2x0 C. When the superconducting material has a typical BSCCO-2223 composition, bonding temperature should not exceed 842xc2x0 C.; and should not be around 780xc2x0 C., at which rapid formation of detrimental secondary phases occurs. With this example, the preferred bonding temperature is in the range of 815xc2x0 C. to 825xc2x0 C.
More preferably, the bonding is performed at a temperature below 700xc2x0 C. in an inert atmosphere or in a vacuum to avoid oxidation of the metal sheaths during the bonding process. Although a diffusion time of several hours will be required to achieve adequate bonding it is preferred to avoid excessively long diffusion times as they tend to produce too much sintering of the superconductor material.
Preferably, the diffusion bonded stack of tapes is rolled to reduce overall thickness and to strengthen the bonding.
According to another aspect of the invention there is provided a method of making a composite superconducting tape, the method comprising the steps of:
stacking a multiplicity of constituent monofilamentary or multifilmentary superconducting tapes together;
heat treating the stack of superconducting tapes to promote diffusion bonding; and
rolling the diffusion bonded stack.
According to another aspect of the invention there is provided a composite superconducting tape constructed from a plurality of elongate superconducting tapes which each include at least one major surface, the composite superconducting tape including:
a diffusion-bonded stack of the plurality of superconducting tapes in which all elongate components extend longitudinally and in which at least one of the major surfaces is exposed; and
a compatible metal tape that is bonded to the exposed major surface.
Preferably, the plurality of superconducting tapes each include at (east one superconductive filament and an outer casing of pre dominantly silver for both containing the filaments and defining the major surface, the metal tape including a first surface for abutting the at least one exposed major surface and a second surface opposite the first surface. More preferably, the metal tape is silver and the distance between the second surface and the closest filament of the adjacent superconducting tape is at least 10 xcexcm.
Preferably also, the stack includes a second exposed major surface and the composite superconductor tape includes a second compatible metal tape which is bonded to the second exposed major surface. More preferably, the metal tapes differ in at least one characteristic. Even more preferably, the differing characteristic is chosen from the set consisting of: thickness; strength; rigidity; width; and coefficient of thermal expansion.
In a preferred form, the composite superconducting tape includes a second diffusion-bonded stack having a plurality of superconducting tapes, wherein the two stacks are maintained in a substantially parallel configuration.
Preferably, the metal tape is diffusion bonded to the exposed major surface.
Unless the context clearly requires otherwise, throughout the description and the claims, the words xe2x80x98comprisexe2x80x99, xe2x80x98comprisingxe2x80x99, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of xe2x80x9cincluding, but not limited toxe2x80x9d.